1. Field of the Invention
The present invention relates to electrically controllable systems having variable optical properties, and more specifically to glazing units in which the light scattering and/or the light transmission can be modified due to the effect of a suitable electrical supply.
2. Discussion of the Background
There is in fact a growing demand for so-called xe2x80x9csmartxe2x80x9d glazing, certain properties of which can be modulated as required. Controlling or modifying the level of light scattering of glazing thus allows the degree of visibility through the glazing to be controlled, especially so that it is transparent or, on the contrary, scattering, thus preventing individuals or objects on the other side of the glazing from being identified. There are various applications for such glazing: it is thus possible to consider equipping the internal partitions between rooms in a building, especially in offices, or between two areas/compartments of means of land, air or maritime locomotion with such glazing, or for equipping shop windows or display cabinets, or any type of container. In general, such glazing can also be used for equipping any window in a building or in means of locomotion (windows on trains, cabin portholes on boats or cabin windows on aircraft).
At the present time there are various families of functional systems having electrically controllable light scattering/transmission (hereafter referred to as xe2x80x9cfunctional systemsxe2x80x9d).
A first family of functional systems is known by the term liquid-crystal glazing. This is based on the use of a film based on a polymer material and placed between two conducting layers, droplets of liquid crystals, especially nematic liquid crystals having positive dielectric anisotropy, being dispersed in the said material. When a voltage is applied to the film, the liquid crystals orient in a preferred direction, thereby allowing vision. With no voltage applied, the crystals not being aligned, the film becomes diffusing and prevents vision. Examples of such films are described especially in European Patent EP 0,238,164 and U.S. Pat. Nos. 4,435,047, 4,806,922 and 4,732,456. This type of film, once laminated and incorporated between two glass substrates, is sold by Saint-Gobain Vitrage under the brand name xe2x80x9cPriva-Litexe2x80x9d in fact, it is possible to use any device based on liquid crystals known as xe2x80x9cNCAPxe2x80x9d (Nematic Curvilinearly Aligned Phase) or xe2x80x9cPLDCxe2x80x9d (Polymer Dispersed Liquid Crystal) crystals.
Another family is that commonly referred to by the term optical valve: this generally involves films comprising a polymer matrix, optionally crosslinked, in which microdroplets are dispersed, these microdroplets containing particles which have the property of moving in a preferred direction under the action of an electric or magnetic field. Depending in particular on the potential applied to the terminals of the conducting layers placed on either side of these films and on the concentration and nature of the orientable particles, the films have variable optical properties. For example, Patent WO 93/09460 discloses an optical valve based on a film comprising a crosslinkable polyorganosiloxane matrix and inorganic or organic orientable particles, more particularly light-absorbing particles such as polyiodide particles. When a voltage is applied to the film, the particles intercept the light much less than when a voltage is not applied: this system therefore makes it possible to obtain glazing with variable light transmission, generally associated with light scattering that can also be varied.
Whether optical valves or liquid-crystal systems are used, these systems are usually in the form of a polymer film. To supply it with electrical power, it is usually placed between two electrically conducting layers, which in particular are transparent, for example made of doped metal oxide of the tin-doped indium oxide (ITO) type or the fluorine-doped tin oxide (F:SnO2) type. Furthermore, the film with its two conducting layers is usually provided on at least one of its sides, and thus each of its sides, with a carrier/protector substrate. This is generally transparent. It may be chosen so as to be rigid or semi-rigid and made of inorganic or organic material, for example made of glass, or an acrylic polymer of the polymethyl methacrylate (PMMA) type. It may also be flexible, especially made of polyethylene terephthalate PET it is thus possible to have a structure of the PET/ITO/functional film/ITO/PET type, which is in the form of a flexible sheet that can be easily handled. This assembly (polymer+electrically conducting layers+at least one carrier substrate) can then be laminated to at least one transparent rigid substrate of the glass type using at least one joining layer of organic polymer of the polyvinyl butyral PVB or ethylene-vinyl acetate EVA type or certain polyurethanes PU.
Attempts have been made to add other functionalities to liquid-crystal glazing, especially in order to be able to vary not only its level of light scattering but also its level of light transmission, by making use of dyes. Patents EP 0,156,615 and EP 0,121,415 thus describe, for example, dyes of the pleochroic type, which are dissolved in the liquid crystal droplets, thereby making it possible to obtain glazing which is both dark/colored and diffusing when no voltage is applied, and both clear and non-diffusing when voltage is applied. Thus, a xe2x80x9cscreenxe2x80x9d effect can be obtained, making the use of such glazing more attractive in the case of outside applications, for example as glazing for building facades or as car sunroofs.
However, these outdoor applications subject the glazing to considerable stress, and it has turned out that the functional systems with additional dyes (such as liquid-crystal glazing) or functional systems having polarizing particles which themselves provide a dye effect, especially by being of the dichroic type, have tended to have a markedly shorter lifetime than those which were devoid thereof, this being even more striking when they were used on the outside.
The object of the invention was therefore to remedy this drawback, by improving functional systems having electrically controllable light scattering/transmission, most particularly those using dichroic dyes or polarizing particles with a dye effect, the improvement being aimed especially at increasing their lifetime and increasing their durability.
The subject of the invention is firstly an electrically controllable system having variable light scattering/transmission, which comprises a functional film provided with electrically conducting layers. This film has so-called xe2x80x9cactivexe2x80x9d elements either in the form of particles, in particular polarizing particles (optical-valve system) or in the form of liquid crystals (liquid-crystal system) which are associated with dichroic dyes, and in suspension in a medium. The invention also provides the system with one or more means for preventing/compensating for degradation by photoreduction of at least some of the active elements, especially that of the dichroic dyes (in the case of liquid-crystal systems) or of the polarizing particles themselves (in the case of optical-valve systems).
This is because the invention has discovered the mechanism which caused the systems to age prematurely, this being a photoreduction mechanism which tended to irreversibly degrade the dichroic dyes and the polarizing particles when the systems were subjected to intense and/or prolonged ultraviolet radiation. The solution therefore consisted in using means for combating this degradation, these means being aimed either at preventing this photoreduction or, preferably, at allowing the dyes, in reduced/degraded form, to be as it were xe2x80x9cregeneratedxe2x80x9d by permanently re-oxidizing them, thus xe2x80x9ccompensatingxe2x80x9d for the photochemical reduction that they undergo.
According to a first variant, the means preventing degradation by photoreduction comprises the use of at least one of the two electrically conducting layers in the form of a multicomponent layer comprising a conducting layer based on a doped metal oxide, the said layer being physically isolated from the functional film by at least one barrier layer of a different chemical nature.
The term xe2x80x9cmulticomponentxe2x80x9d layer should be understood to mean a superposition of at least two layers made of different materials which together form the electrically conducting xe2x80x9clayerxe2x80x9d in the sense of the invention. The doped metal oxide in question may be doped tin oxide, especially fluorine-doped tin oxide F:SnO2, tin-doped indium oxide ITO or zinc oxide doped, for example, with aluminum, all these materials being well known.
The term xe2x80x9cphysically isolatedxe2x80x9d should be understood to mean that there is no direct contact between the functional film and the layer of doped metal oxide. The inventors realized in fact that it is this direct contact which seemed to be responsible for the premature degradation of the system, one hypothesis explaining this being that the metal oxide would have photocatalysis properties sufficient to cause degradation of the film which is contiguous with it. The solution provided by the invention therefore consisted in making use of a barrier layer in order to be able to continue to use electrically conducting layers made of doped metal oxide without suffering the drawbacks thereof (this is because these layers are, moreover, highly advantageous since their manufacture, especially by pyrolysis of the CVD type or by vacuum sputtering, is well controlled and they provide the best balance between electrical conductivity and transparency) This barrier layer may be essentially metallic, especially made of nickel or chromium, or a nickel-chromium alloy. In this case, the layer is also conducting from an electrical standpoint, but it is preferably chosen to be relatively thin in order to prevent the appearance of the glazing being optically modified too much, so that the level of transparency of the multicomponent conducting layer is retained.
The barrier layer may thus be chosen to be made of a material with little or no conductivity, such as silicon or derivatives of silicon, especially silicon dioxide. Here again, it is advantageous to choose it so that the thickness sufficient for the above mentioned physical isolation is as small as possible. Silicon nitride Si3N4, silicon oxinitride SiON or aluminum nitride AlN may also be chosen. The silicon oxide is, in a non-limiting manner, the preferred example of the family of dielectrics based on non-photocatalytic oxides.
The barrier layers having all substantially no photocatalytic activity are generally chosen to have a thickness of less than 10 nm, especially less than 5 nm, for example between 0, 5 and 3 nm. (It is also possible to superpose two barrier layers of different nature, if desired). A more radical solution consists in not using electrically conducting layers based on doped metal oxide, at least on one side of the functional film, and in replacing it with a conducting material which is not based on an oxide.
Embodiments of this variant may be a system (fox example deposited between two flexible substrates or between a flexible substrate and a rigid substrate) comprising:
1) ITO
2) NiCr or SiO2 barrier layer
3) functional film
4) NiCr or SiO2 barrier layer
5) ITO
with identical xe2x80x9cbilayerxe2x80x9d conducting layers on each side of the functional film.
One of the two bilayers may be replaced by a metal monolayer, for example a gold-titanium alloy, or a metal multilayer, for example NiCr/Au/NiCr. It is also possible to combine the use of a multicomponent layer on one side of the functional film with a standard conducting layer made of doped metal oxide: a single conducting layer according to the invention already gives the system a longer lifetime.
The two bilayers may also be replaced with this metal monolayer or multilayer. The means preventing degradation then comprises selecting at least one essentially metallic electrically conducting layer (by therefore completely avoiding the use of a material made of doped metal oxide, in at least one of the two conducting layers). It has in fact been found that substituting the usual doped-oxide electrically conducting layers with metallic layers, or at least one of the two, considerably increased the lifetime of the system, although the precise reasons for this are not fully understood.
This variant has the advantage of being simple to implement: the techniques available for depositing thin metallic-type layers allow their parameters, especially their chemical nature, density and thickness, to be effectively controlled so as to obtain the desired properties, most particularly in this case a sufficient level of electrical conductivity combined with transparency. Mention may more particularly be made of the technology of deposition by magnet-field-enhanced sputtering.
The metal layer may advantageously be based on silver or gold, or generally on a noble metal or alloy containing at least one of them. In this case, it is desirable for the layer to be provided on at least one of its sides (preferably on both sides) with a thinner protective layer (and/or having an xe2x80x9canchoringxe2x80x9d role), especially one also made of metal of the Ni alloy, of the NiCr or Iconel type, or steel type.
The metal layer may also be based on a gold-titanium alloy. This alloy is particularly advantageous since titanium appears to be able to stabilize gold, thereby making the above-mentioned protective layers optional, and even unnecessary.
According to a second variant (which is an alternative to or a combination with the first variant), the means preventing degradation comprises bringing the film prodded with its electrically conducting layers into contact, on at least one of its sides, with a sheet based on an oxygen-permeable polymer material. It has been found that the lifetime of the system could be considerably increased in this way. In a simplified/pictorial manner, it is just as if this sheet served as an oxygen xe2x80x9creservoirxe2x80x9d for the film or allowed oxygen coming from an oxygen source to pass through it, this having an oxidizing effect which xe2x80x9ccompensatesxe2x80x9d for the reducing effect of the ultraviolet radiation on the dyes.
Advantageously, the sheet based on a polymer material has an oxygen permeability of at least 10 and especially at least 20 or 40 cm3/cm2/mm/s/cmHgxc3x9710xe2x88x9210 measured in accordance with the ASTM-D1434 standard. It may especially be based on one or more polymers belonging to the polycarbonate family, having a permeability of approximately 55 cm3/cm2/mm/s/cmHgxc3x9710xe2x88x9210 (especially those of the pliant/flexible type).
By way of comparison, the polyethylene terephthalate (PET) sheets normally used have a permeability of 0.3 cm3/cm2/mmls/cmHgxc3x9710xe2x88x9210.
The term xe2x80x9csheetxe2x80x9d should be taken in the wide sense: it may be a carrier substrate for the functional film and for a protective sheet and for a sheet allowing it to be laminated to a rigid substrate and, more generally, allowing it to be incorporated into glazing units of the multiple-glazing and/or laminated-glazing type. In general, this sheet is therefore pliant/flexible.
In fact, it is preferable for the electrically conducting layer(s) placed between the functional film and the oxygen-permeable sheet to have a degree of xe2x80x9cporosityxe2x80x9d allowing oxygen transfer. Likewise, provision may be made to trap enough oxygen in the permeable sheet so that it can gradually release oxygen towards the functional film. It is therefore desirable for this film to be in at least partial contact with a renewable oxygen source. If the assembly (functional film+electrically conducting layers+oxygen permeable sheet(s)) is intended to be incorporated in a glazing unit so that the permeable sheet(s) is (are) gripped between the film and for example, a rigid substrate which is not permeable to oxygen (laminated-glazing structure), it is possible to envisage providing the permeable sheet with peripheral venting means in order to ensure replenishment of the oxygen in the sheet, for example by means of capillaries.
If the aforementioned assembly is intended to be incorporated in a multiple-glazing unit of the insulating double-glazing or varietally dynamic glazing type, the permeable sheet may be placed so as to bring it into contact with the intermediate gas layer, of the air-layer type, by preferably providing an air replenishment system of the xe2x80x9cbreathablexe2x80x9d glazing type.